/Engine/Source/Runtime/Slate/Public/Framework/MultiBox/MultiBoxBuilder.h
Pushes a new extender onto the stack. This extender will be used for all subsequently-added multiblocks, until the extender is popped.
void PushExtender ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [FExtender](API\Runtime\Slate\Framework\MultiBox\FExtender) > InExtender )